I. Field of the Prior Art
This invention relates to a cast aluminum alloy product suitable for making can stock, and to a process for making the product. It also relates to an alloy sheet product suitable for making cans, and to a process for making the product.
II. Description of the Prior Art
Aluminum beverage cans are made from sheet-form alloys such as alloys designated as AA3004, AA3104 and similar alloys containing Mg, Mn, Cu, Fe and Si as principal alloying elements. The sheet is generally made by direct chill (DC) casting an ingot (typically 500 to 750 mm thick) of the desired composition, homogenizing the ingot at temperatures of 580 to 610.degree. C. for periods of 2 to 12 hours, and hot rolling the ingot (employing a mill entry temperature of about 550.degree. C.), thereby reducing it to re-roll sheet of about 2 to 3.5 mm thick. The re-roll sheet is then cold rolled in one or more steps to the final gauge (0.26 to 0.40 mm). Various annealing steps may be used in conjunction with the cold rolling.
The alloy and processing conditions are selected to give sufficiently high strength, high galling resistance (also referred to as scoring resistance) and low earing to enable fabrication of a can body by drawing and ironing (D&I) operations, and sufficiently high strength retention after paint baking that the finished can is adequately strong. The galling resistance is believed to be related to the presence of intermetallic particles dispersed throughout the ingot, which remain in the final rolled product. It is commonly found that homogenization of a DC cast ingot of suitable composition develops enlarged .alpha.-Al(Fe,Mn)Si (alpha) phase particles which are believed to prevent galling, although there is also evidence (e.g., see Japan patent publication JP 58-126967) to suggest that the formation of (Mn,Fe)Al.sub.6 intermetallic particles during homogenization provides the necessary galling resistance.
The use of continuous casting to produce alloy slab (typically 30 mm in maximum thickness) followed by hot rolling the slab directly (essential in a continuous process without homogenization) to make re-roll sheet has decided advantages in the production of sheet products, in that hot rolling can be carried out without having to reheat a large DC cast ingot. Such a process is disclosed, for example, in U.S. Pat. No. 4,614,224 which teaches the importance of fine alpha phase particles in can performance, but not specifically for imparting galling resistance.
However, when such a continuous process is used as the initial step in producing a final sheet suitable for can production, the properties required for modern can production cannot all be met in the way that DC cast material meets these requirements. Such continuously cast material generally has excessive earing and excessive galling or scoring during can making operations.
Strip cast can body stock material has been produced with large particles distributed through the slab, but only by incorporating a homogenization step prior to hot rolling, as in DC casting.
British Patent GB 2 172 303 discloses strip cast can stock material in which alpha phase particles are generated and grown to a suitable size to prevent galling using homogenization of the cast strip.
U.S. Pat. No. 4,111,721 discloses strip cast material in which homogenization is also used to grow (Mn,Fe)Al.sub.6 particles above a size suitable to prevent galling.
Both of these continuous casting processes have the disadvantage of requiring an homogenization step to achieve the desired effect. This must be carried out on a coil, and temperature control is critical to avoid excessive oxidation of the coil and adhesion of the coil layers to each other. Furthermore the addition of such a step removes much of the cost advantage present in a continuous process.
In all previously developed processes which generate large intermetallics suitable for prevention of scoring, the process generates large intermetallics throughout the strip, whereas the large intermetallics are of value in preventing galling only at the surface of the strip. Elsewhere they may be detrimental.
There is a need therefore for a strip making process based on a continuous casting process which is capable of producing a strip having properties meeting modern can and can fabrication requirements, which is made cost effective through the elimination of certain process steps (such as homogenization) previously considered essential.